The present invention relates to a novel technique for initiating free radical processes. Numerous chemical processes of great commercial importance are initiated by the use of free radical initiators. Examples of such chemical phenomena include oxidation, photolysis, and polymerization processes wherein free radicals act either as initiators or intermediates therein. A particularly highly useful free radically initiated process is the polymerization of monomers containing ethylenic unsaturation. Examples include the free radically initiated polymerization of vinyl containing monomers such as styrene, methylmethacrylate, etc. as well as dienes particularly conjugated dienes such as butadiene or isoprene and crosslinking reactions involving the free radically initiated polymerization of a polymeric component containing ethylenic unsaturation optionally in the presence of a coreactant.
Previously known free radical initiators have not been thermally stable. Upon incorporation of the free radical generator with the free radical sensitive compound the desired reaction would be initiated. While to a certain extent control of the process could be obtained by cooling the resulting mixture, there is no known technique for the delayed initiation of free radical processes.
In certain applications it would be desirable to provide a free radical sensitive compound which may be formulated into a resulting product or merely retained in a shelf stable condition until later when upon the occurrence of a suitable activating event the desired chemical reaction is allowed to take place.
In particular it would be desirable to provide a crosslinkable polymeric composition which is thermally stable and may be processed under normal melt extrusion or other processing conditions but may at a later time upon the occurrence of a suitable activating event be crosslinked through free radical initiated polymerization.
In Canadian Patent No. 738,500; an ethylene bismaleimide and an N,N'-meta phenylene bismaleimide are disclosed for use as vulcanizing agents in the crosslinking of various unsaturated rubbers including homopolymers and heteropolymers of conjugated dienes, e.g., polybutadiene and butadiene/styrene copolymers, and polyurethane rubbers. Such vulcanizing agents were thermally activated.
In U.S. Pat. No. 3,674,486, photoresist materials comprising a photosensitizer, a crosslinking agent and a rubber-like constituent are disclosed. Suitable crosslinking agents disclosed included multifunctional olefins, such as triallyl cyanurate, pentaerthritol triacrylate, and polyvinyl cinnamate.
In U.S. Pat. Nos. 4,079,041, 4,163,097, 4,158,730 and 4,158,731 there are disclosed certain organic polymers which can be crosslinked under the action of light having a molecular weight of at least 1,000 and containing as light sensitive groups di-substituted maleimide functionality.
In U.S. Pat. No. 2,925,407, the curing of high molecular weight substantially unsaturated polymers by the use of free-radical generators such as organic peroxide and free-radical exceptors such as bismaleimide compounds, among others, is disclosed. Such a curing operation is thermally activated.
The teachings of the above publications are incorporated in their entirety by reference thereto.
Despite advances in the art utilizing photolytically active curing agents, there remains a desire to prepare compositions that may be readily crosslinked but are insensitive to curing prematurely under the action of heat. Compositions which are not thermally stable cannot easily be employed in thermal processing operations such as melt extrusion, extrusion coating, melt mixing and other high temperature processes. Alternative techniques such as solvent deposition or other suitable technique must be employed to manufacture and apply thermally sensitive compositions. However, solvents are both expensive and undesirable from the standpoint of atmospheric emissions and combustion hazard. Moreover, thermally unstable compositions and materials prepared therefrom must be subsequently treated in a manner so as to minimize the effects of thermal crosslinking. For example, flexible or flexographic printing plates that are subject to thermal crosslinking should be maintained at reduced temperatures in order to impart practical lifetimes. The need for refrigeration of such materials has limited the extent to which they have been commercially exploited.